Various interactive haptic technologies exist, which provide a user or users with tactile information or feedback, often in combination with visual information displayed on an interactive screen. For example, previous haptic feedback devices include pins moving to physically change a deformable surface. A pen connected to an articulated arm may be provided, as in the SensAble PHANTOM device. Alternatively, a user may wear, for example in the form of a glove, one or more actuators which are activated to provide haptic feedback to a user. However, in each of these technologies, a user requires physical contact with a deformable surface, a pen, or a specially adapted glove. Such requirements reduce the usability and spontaneity with which a user may interact with a system.
Tactile sensations on human skin can be created by using a phased array of ultrasound transducers to exert an acoustic radiation force on a target in mid-air. Ultrasound waves are transmitted by the transducers, with the phase emitted by each transducer adjusted such that the waves arrive concurrently at the target point in order to maximise the acoustic radiation force exerted.
Ultrasound haptic feedback systems create a vibrotactile sensation upon the skin of a user of the system. The focussed ultrasound creates enough force at the point of intersection to slightly displace the skin of a user. Typically, ultrasound haptic feedback systems use ultrasound with a frequency at or above 40 kHz, which is above the threshold for receptors in the skin to feel. Therefore, a user can only detect the onset and cessation of such focussed ultrasound. In order to provide a sensation that is detectable by the receptors in skin, the focussed ultrasound is modulated at a lower frequency, within the detectable range of the receptors. This range is typically from 1 Hz to 500 Hz.
A side effect of the modulation is that the ultrasound breaks down and creates a sound at the modulation frequency. Therefore, when creating tactile feedback with a 200 Hz modulation frequency, a 200 Hz sound is also produced. This audible sound may be annoying to users and is a barrier to ultrasound haptic technology being adopted.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved haptic feedback system.